Two Pi R

Although many people are excited about the hardware-hacking possibilities
with the Raspberry Pi, one of the things that interests me most is
the fact that it is essentially a small low-power Linux server I can
use to replace other Linux servers I already have around the house. In
previous columns, I've talked about using the Raspberry Pi to replace
the server that controls my beer fridge and colocating a Raspberry Pi in
Austria. After I colocated a Raspberry Pi in Austria, I started thinking
about the advantages and disadvantages of using something with so many
single points of failure as a server I relied on, so I started thinking
about ways to handle that single point of failure. When you see "Two Pi
R", you probably think the R stands for the radius for a circle. To me, it
stands for redundancy. I came to the conclusion that although one Pi isn't
redundant, two Pi are.

So, in this article, I'm building the foundation for setting up redundant
services with a pair of Raspberry Pis. I start with setting up a
basic clustered network filesystem using GlusterFS. In later articles,
I'll follow up with how to take advantage of this shared storage to set
up other redundant services. Of course, although I'm using a Raspberry Pi
for this article, these same steps should work with other hardware as well.

Configure the Raspberry Pis

To begin, I got two SD cards and loaded them with the latest version
of the default Raspberry Pi distribution from the official Raspberry Pi
downloads page, the Debian-based Raspbian. I followed the documentation
to set up the image and then booted in to both Raspberry Pis while
they were connected to a TV to make sure that the OS booted and that
SSH was set to start by default (it should be). You probably also will
want to use the raspi-config tool to expand the root partition to fill
the SD card, since you will want all that extra space for your redundant
storage. After I confirmed I could access the Raspberry Pis remotely,
I moved them away from the TV and over to a switch and rebooted them
without a display connected.

By default, Raspbian will get its network information via DHCP; however,
if you want to set up redundant services, you will want your Raspberry
Pis to keep the same IP every time they boot. In my case, I updated my
DHCP server so that it handed out the same IP to my Raspberry Pis every
time they booted, but you also could edit the /etc/network/interfaces
file on your Raspberry Pi and change:

Of course, modify the networking information to match your personal
network, and make sure that each Raspberry Pi uses a different IP. I
also changed the hostnames of each Raspberry Pi, so I could tell them
apart when I logged in. To do this, just edit /etc/hostname as root and
change the hostname to what you want. Then, reboot to make sure that each
Raspberry Pi comes up with the proper network settings and hostname.

Configure the GlusterFS Server

GlusterFS is a userspace clustered filesystem that I chose for this
project because of how simple it makes configuring shared network
filesystems. To start, choose a Raspberry Pi that will act as your
master. What little initial setup you need to do will be done from the
master node, even though once things are set up, nodes should fail over
automatically. Here is the information about my environment:

Before you do anything else, log in to each Raspberry Pi, and install the
glusterfs-server package:

$ sudo apt-get install glusterfs-server

GlusterFS stores its files in what it calls bricks. A brick is
a directory path on the server that you set aside for gluster to
use. GlusterFS then combines bricks to create volumes that are accessible
to clients. GlusterFS potentially can stripe data for a volume across
bricks, so although a brick may look like a standard directory full of files,
once you start using it with GlusterFS, you will want to modify it only
via clients, not directly on the filesystem itself. In the case of the
Raspberry Pi, I decided just to create a new directory called /srv/gv0
for my first brick on both Raspberry Pis:

$ sudo mkdir /srv/gv0

In this case, I will be sharing my standard SD card root filesystem,
but in your case, you may want more storage. In that situation, connect a USB
hard drive to each Raspberry Pi, make sure the disks are formatted, and
then mount them under /srv/gv0. Just make sure that you update /etc/fstab
so that it mounts your external drive at boot time. It's not required
that the bricks are on the same directory path or have the same name,
but the consistency doesn't hurt.

Kyle Rankin is a director of engineering operations in the San Francisco Bay Area, the author of a number of books including DevOps Troubleshooting and The Official Ubuntu Server Book, and is a columnist for Linux Journal.

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I believe it's not possible to put your root to GlusterFS but I'm planning to install ownCloud on GlusterFS. Have you any idea how fast connection it need for replication to work? I would have one server at the office and another one at home and the connection is 60 / 10 Mbit/s between them.

The geo-replication feature was created specifically for those cases where you have a WAN link or some other unreliable network connection. It's eventually consistent, as opposed to the default replication that's synchronous and strongly consistent.

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